1. Field of the Invention
This invention relates to a multi-piece solid golf ball of a structure having at least four layers, and more particularly, to such a multi-piece solid golf ball which is improved in flight performance, hitting feel, and controllability, and rolls straight on the green.
2. Prior Art
Golf balls of various structures are currently on the market. Among others, two-piece solid golf balls having a rubber based solid core encased in a cover typically of ionomer resin and thread-wound golf balls produced by winding thread rubber around a solid or liquid center and enclosing the center with a cover are commonly used in competitions.
The two-piece solid golf balls are used by many ordinary golfers because of superior flying performance and durability although they have the drawbacks including a very hard feel upon hitting and less controllability because of quick separation from the club face upon impact.
To improve the hard hitting feel of solid golf balls, various two-piece solid golf balls of soft type were proposed. In general, many soft type two-piece solid golf balls use soft cores. Softening the core invites not only a lowering of restitution which leads to poor flight performance, but also a substantial loss of durability. Then the flight performance and durability characteristic of two-piece solid golf balls are not maintained, sometimes giving rise to the problem that the golf balls are practically unacceptable.
Therefore, there is a desire to have a solid golf ball which can be improved in feel and controllability at no sacrifice of flight distance and durability. One of such proposals is a three-piece solid golf ball of three layer structure having an intermediate layer interposed between the inner sphere and the cover.
Such three-piece solid golf balls proposed heretofore include, for example,
(1) a three-piece solid golf ball comprising a solid core consisting of a relatively soft, small diameter inner sphere and a harder intermediate layer surrounding the inner sphere wherein the percent area of contact with the club face upon hitting is specified (Japanese Patent Publication (JP-B) No. 55077/1992, Japanese Patent Application Kokai (JP-A) No. 80377/1989, etc.); PA1 (2) a three-piece solid golf ball comprising a solid core (or inner sphere), an inner cover surrounding the core as an intermediate layer, and an outer cover formed on the surface of the inner cover wherein the diameter and specific gravity of the solid core, the gage, specific gravity and JIS-C hardness of the inner cover, and the gage of the outer cover are specified, especially the JIS-C hardness of the inner cover is specified relatively high (JP-A 24084/1995 etc.); and PA1 (3) a three-piece solid golf ball wherein the diameter and specific gravity of the solid core, the gage, specific gravity and JIS-C hardness of the intermediate layer (or inner cover), and the gage of the outer cover are specified, especially the JIS-C hardness of the intermediate layer is specified relatively high (JP-A 24085/1995). PA1 (1) a multi-piece solid golf ball comprising a core including an inner sphere and at least one enclosure layer surrounding the inner sphere and a cover consisting of an inner cover surrounding the core and an outer cover surrounding the inner cover, characterized in that said inner sphere has a hardness expressed by a distortion of 3 to 8 mm under a load of 100 kg, said enclosure layer has a surface hardness higher than the surface hardness of the inner sphere in Shore D, said inner cover has a Shore D hardness of at least 55, said outer cover has a Shore D hardness of 35 to 53, and said ball has a weight of 44.5 to 45.93 grams and an inertia moment of at least (1.52.times.ball weight (gram)+12.79) g.multidot.cm.sup.2 ; PA1 (2) a multi-piece solid golf ball according to (1) wherein said inner sphere is formed of a rubber base material and has a diameter of 20 to 37 mm, and said core has a diameter of 32 to 41 mm; PA1 (3) a multi-piece solid golf ball according to (1) or (2) wherein the inner cover has a gage of 0.5 to 3 mm, the outer cover has a gage of 0.3 to 3 mm, and the difference in Shore D hardness between the inner cover and the outer cover is at least 5; PA1 (4) a multi-piece solid golf ball according to any one of (1) to (3) wherein at least one layer of the outer cover, the inner cover and the enclosure layer has a high specific gravity inorganic filler blended therein; and PA1 (5) a multi-piece solid golf ball according to any one of (1) to (4) wherein the outer cover is formed of a thermoplastic polyurethane elastomer.
However, the proposal relating to the three-piece solid golf ball (1), in which the hardness of the cover is not definitely specified, would provide insufficient restitution if a golf ball having a relatively soft or low hardness cover is formed. If a golf ball having a relatively hard or high hardness cover is formed, the hitting feel upon approach shots causing smaller deformation becomes hard because the intermediate layer encased in the cover is harder than the solid core.
Also, the three-piece solid golf ball (2) offers a soft hitting feel, but is difficult to control because of the hard outer cover.
Further, the three-piece solid golf ball (3), in which the core that mostly affects feel and restitution is made relatively soft, provides insufficient restitution and fails to travel a long distance as long as the hitting feel is fully soft. Inversely, in the core hardness range ensuring sufficient restitution, the hitting feel is hard because the intermediate layer is also hard, and low-head speed players cannot provide the ball with sufficient deformation to fly a long distance.
Meanwhile, with respect to the above-mentioned problems of solid golf balls, it was recently proposed to achieve an improvement by increasing an inertia moment. For example, JP-B 48473/1992 proposes a solid golf ball whose inertia moment is controlled by adding fillers to the cover stock and the core stock, for thereby increasing the flight distance without deteriorating the durability and appearance of the cover surface.
However, this proposal, in which the hardness of the core is not definitely specified, suffers from the problem that the use of a hard core results in a hard hitting feel whereas the use of a soft core softens the hitting feel at the sacrifice of restitution and flight performance. The ball is less easy to control because of the hard outer cover.